1. Field of the Invention
The present invention relates to a transistor circuit for driving various elements, and more particularly to a transistor circuit having its operation and inspection test facilitated.
2. Description of the Related Art
Transistors are commonly used for controlling supply and output of electric current in display devices and sensors.
An example of a flat display panel is an organic EL display panel. An organic EL display panel is configured by arranging, in a matrix, a large number of pixels each composed of an organic EL element. One method for driving an organic EL element is the active matrix method in which a switching transistor is provided for each pixel, and display of each pixel is controlled by controlling the corresponding transistor.
FIG. 5 shows an example circuit configuration of a pixel in an organic EL panel employing conventional thin film transistors (TFT). An organic EL panel is configured by arranging such pixels in a matrix.
A gate line GL extending in a row direction is connected to the gate of a first TFT 10, which is an n-channel thin film transistor selected by the gate line GL. The drain of the first TFT 10 is connected to a data line DL extending in a column direction. The source of the first TFT 10 is connected to a storage capacitor CS, which in turn is connected on the other terminal with a capacitor line SL serving as a low-voltage power source. An intermediate point in the connection between the source of the first TFT 10 and the storage capacitor CS is connected to the gate of a second TFT 40, which is a p-channel thin film transistor. The source of the second TFT 40 is connected to a power line VL, while the drain of the second TFT 40 is connected to an organic EL element EL. The other terminal of the organic EL element EL is connected to a cathode power source CV.
According to this arrangement, when the gate line GL is at level H, the first TFT 10 is turned on. Data provided in the data line DL at that point is stored in the storage capacitor CS. A current in the second TFT 40 is controlled in accordance with the data (potential) stored in the storage capacitor CS. A current then flows in the organic EL element EL in accordance with the current in the second TFT 40, resulting in light emission.
When the first TFT 10 is turned on, a video signal associated with the pixel is supplied through the data line DL. As a result, the storage capacitor CS is charged in accordance with the video signal supplied through the data line DL. A corresponding current then flows in the second TFT 40, thereby executing brightness control of the organic EL element EL. In other words, display of gradation in each pixel is attained by adjusting the gate potential of the second TFT 40 so as to control the current flowing in the organic EL element EL.
In an organic EL panel as described above, a defect may occur during the manufacturing process of the panel in the two TFTs or other components constituting each pixel, resulting in a pixel that cannot execute predetermined display operations. It is difficult to perfectly prevent such point defects which inevitably occur with a predetermined probability. As a defect is visually unnoticeable when it merely generates a dark point in which no display is performed, a defective pixel is typically deactivated by disconnecting its wiring using a laser or the like.
To locate and disconnect defective pixels, it is necessary to conduct an operation test for each pixel. One method for conducting the operation test is to detect secondary electrons reflected by each pixel using a detector so as to confirm operation of the pixel. However, this method is inefficient, requiring a long testing time.
In the process of manufacturing an LCD, an array test is conducted at the point when a pixel electrode is formed, by turning on a switching element to apply a voltage to a storage capacitor and the pixel electrode, and checking the amount of electric charge stored in the storage capacitor. An array tester for this purpose has been developed.
When such an array test performed for an LCD is conducted with respect to an organic EL panel, the output of the first TFT can be tested. However, as one pixel of an organic EL panel includes two TFTs, a test of the second TFT to determine whether an electric current is adequately supplied to the organic EL element (such that light emission results) cannot be inspected.